1. Field of the Invention
The invention relates generally to the field of subterranean remediation and specifically to a method and apparatus for remediation of soil and groundwater which has become contaminated with organic materials.
2. Description of the Prior Art
Subterranean contamination, particularly the organic contamination of soil and groundwater which results from leaking storage facilities or accidental or even purposeful discharge, has become a problem in almost all industrialized areas of the planet.
Industrialized society has historically stored, discharged and disposed of various hazardous substances and waste products to the soil and groundwater over the years as a normal and acceptable form of disposal. In many instances, these discharges have occurred without any knowledge on the part of a facility owner or operator. Frequently, the discharge from tanks, lagoons, drum storage depots and other areas occur over a prolonged period of time. Oftentimes, the discharge results from a failure in the structural integrity of an underground storage tank and/or piping system. The discharged substances contaminate the soil and groundwater thereby causing severe damage to the local environment. As a consequence of these discharges and subsequent contamination, individuals, companies and governments are forced to expend a great deal of time and financial resources in remediation efforts.
The discharged substances are largely comprised of aromatic and aliphatic organic compounds refined from petroleum hydrocarbons. Frequently, these substances include halogenated organic substances and solvents which may also present a significant carcinogenic risk. These discharged substances have shown the ability to migrate to great depths beneath the surface of the soil and are, therefore, difficult to remediate by conventional methodologies. Examples of such substances discharged into the soil and groundwater include gasoline, fuel oil, motor oil, polychlorinated biphenyl (PCB), benzene, toluene, ethyl benzene and xylene. These latter four substances (Benzene, Toluene, Ethyl Benzene and Xylene) are the subject of BTEX (which is an Acronym for Benzene, Toluene, Ethyl Benzene and Xylene) testing. The BTEX measurement is often used as a quantitative determination of soil and groundwater contamination where a discharge of gasoline and/or fuel oil has occurred.
A number of remediation techniques have been used over the years and are shown in the prior art. Typical remediation techniques to remove contaminated soil from the environment include excavation of the contaminant and either: 1) depositing the excavated materials in a landfill as a temporary storage of the waste ("truck and haul" approach), or 2) transporting the excavated material to an incinerator and incinerating the excavated material. Some sites have been found to be so heavily contaminated that the construction of an incinerator on the site is justified. Regardless of where the incineration takes place, the incineration of contaminated materials can degrade air quality and further risk exposure of these contaminants to the environment at large. The "truck and haul" approach to soil remediation, generally consists of excavating the contaminated site and then hauling the contaminated soil away by truck to a landfill. The truck and haul approach is by far the most expensive method of soil remediation and is also the method least preferred by regulatory agencies. Placing contaminated soil in a landfill carries the additional risk of having to remediate the landfill at some future date, or perhaps exposing the owner of the landfill to additional liability should the landfill become subject to coverage under the Superfund Act.
Groundwater remediation consists mainly of "pump and treat" procedures whereby contaminated groundwater is pumped from underground to the surface where that groundwater is treated thereby remediating the contaminants. Remediated groundwater is then returned underground. Such a procedure is usually expensive and can require years to perform. Additionally, pump and treat groundwater remediation procedures may never fully return the groundwater to an acceptable condition to serve as a source of drinking water. Pump and treat groundwater remediation methods and their associated technologies are inherently inefficient; therefore, more advanced methodologies are required to reduce costs and more completely and safely effect treatment of groundwater.
More recently, however, the prior art has focused on providing in-situ remediative techniques, particularly employing bioremediation and in-situ oxidation using strong oxidizing agents, such as hydrogen peroxide.
Bioremediation technology has been of great interest in recent years; however, its effect on the treatment of contaminated soil and groundwater has been limited. For example, U.S. Pat. No. 4,749,491 (issued to Lawes et al on Jun. 7, 1988, hereinafter the Lawes '491 patent) teaches an in-situ process for the aerobic microbiological decomposition of chlorinated aliphatic hydrocarbons. Chlorinated aliphatic hydrocarbons such as 1,1,1- and 1,1,2-trichloroethane, 1,1- and 1,2-dichloroethane, trichloroethylene, chloroform, 1,1-dichloroethylene, cis- and trans-1,2-dichloroethylene, and vinyl chloride which have been reported as not being biodegradable under aerobic processes, can be eliminated through the process disclosed by the '491 Lawes patent by treatment of the contaminated water and soil with hydrogen peroxide and nutrients and indigenous microorganisms. In particular, hydrogen peroxide and nutrients in an aqueous solution are fed to the microbial flora. The aqueous solution containing hydrogen peroxide and nutrients is fed through one or more infiltration wells, flows through the subsurface contaminated area and is subsequently pumped from the ground through one or more extraction wells. The flow of the hydrogen peroxide and nutrients through the contaminated area causes indigenous microorganisms to grow consuming the contaminants.
A method for the in-situ remediation of a hydrocarbon-contaminated region of groundwater to destroy or reduce the concentration of hydrocarbon contaminants was shown in U.S. Pat. No. 5,268,141 (issued to Vigneri on Feb. 15, 1994 and referred to herein as the '141 Vigneri patent). This method utilizes a plurality of mutually spaced wells which are drilled into the Earth such that all of the wells intersect the groundwater region. The suitability of the contaminated region to treatment is determined by generating a test flow of hydrogen peroxide solution from one of the wells and monitoring pH changes at each of the other wells as a function of time. Subsequent to the suitability test, a treating flow of hydrogen peroxide solution is applied to one or more of the wells.
The chemistry utilized in the method disclosed in the '141 Vigneri patent dates back to 1898, when a method was developed which oxidized malic acid through the use of hydrogen peroxide and iron salts. This chemistry has been and is still widely used in the waste water industry for treatment of organic wastes (particularly non-biodegradable compounds). The process cleaves and oxidizes organic compounds into successively smaller chained hydrocarbon compounds. The intermediate compounds are mono- and di-carboxylic acids which are non-hazardous, naturally occurring substances that are easily oxidized to carbon dioxide and water during sequential reactions.
Although the chemistry used is well characterized and of known utility, the prior art method taught by the '141 Vigneri patent suffers from several serious infirmities. First, this prior art method does not actively disperse the hydrogen peroxide solution. Rather, this method depends instead upon normal diffusive characteristics of the aquifer. As a result, the hydrogen peroxide solution which is poured into the well has a tendency to remain in close physical proximity to the well and/or the well gravel surrounding the well. This minimizes the effective treatment radius, or oftentimes results in explosive concentrations of hydrogen peroxide being present near the well as the hydrogen peroxide degrades. A resulting explosion could quite literally send hydrogen peroxide, catalyst, groundwater, contaminants and soils shooting skyward and subsequently contaminating surface soils and water. Second, diffusive methods require a long time for the reagents used to diffuse throughout the aquifer thereby adding greatly to the time required to effect treatment of a contaminated area. Additionally, diffusive methods suffer from the fact that they do not actively promote the fracturing of the subsurface region, further limiting the effective treatment radius of the method.
Further limitations of diffusion only methods such as those taught by the '141 Vigneri patent include: the requirement for a relatively large number of wells because of the small effective treatment radius; and the extended time required to effect treatment due to the limitation of both the hydrogen peroxide concentrations used to 10-35% by weight and the flow rate of the treating solution to approximately 1-6 gallons per minute per well so as to reduce the possibility of explosions. Still further, while diffusive methods such as those taught by the '141 Vigneri patent have shown limited effectiveness in treating aquifers, there appears to be no evidence that such diffusive methods are of any value in remediating contaminant-laden subterranean soils.
Therefore, a continuing need exists in the art for a method, including accompanying apparatus, to remediate contaminated soil and groundwater more efficiently, reliably, economically and safely than has heretofore occurred with conventional methods.